An antenna for transmitting and receiving an electromagnetic wave using a magnetic material (hereinafter referred to merely as “magnetic antenna”), in which a magnetic field component coming from the outside is allowed to pass through the magnetic material, around which a coil of a conductive wire is wound, to convert the magnetic field component into a voltage (or current) induced by the coil, has been widely used in small-sized radios and TVs. Such a magnetic antenna is also used in a non-contact object identification device called RFID tag which has recently come into use.
To transmit and receive an electromagnetic wave with a higher frequency, a loop antenna free of a magnetic material and including a loop coil having a coil surface parallel to an object to be identified is used in RFID tags. When the frequency is much higher (UHF band or microwave band), an electric field antenna (dipole antenna or dielectric antenna) for detecting an electric field component instead of a magnetic field component is widely used in such devices including RFID tags.
However, such loop antenna and electric field antenna have the following problem. When such an antenna is brought close to a metallic object, an image (mirror effect) is generated on the metallic object. Since the magnetic field of the image has a phase opposite to that of the antenna or since the electric field drops to zero on the metal surface, the sensitivity of the antenna is lost. To overcome the problem, there has been developed a magnetic antenna including a coil having a coil surface with a square or rectangular shape as viewed in a plan view and to be directly attached to a metallic object to be identified such that a cross-section of the coil is perpendicular to a metal surface of the object (Japanese Patent Application Laid-open (KOKAI) No. 2003-317052). Also, a non-contact sensor coil has been proposed in which a metal surface to which a magnetic antenna is attached is positively used to arrange coils in order to cancel a magnetic field parallel to the metal surface and to generate a magnetic field in a direction perpendicular to the metal surface (Japanese Patent Application Laid-open (KOKAI) No. 2003-318634).